Photo acoustic detector with improved signal processing

ABSTRACT

A photo acoustic detector for detecting a concentration of a sample in a mixture includes a light source for producing a light beam for exciting molecules of the sample, and a light modulator for modulating the light beam for generating pressure variations in the sample mixture, where an amplitude of the pressure variations is a measure of the concentration. The detector further includes a detector element for converting the pressure variations into a detector current and a processing section for processing the detector current to generate an output signal representing the concentration. The processing section includes an integrating amplifier for integrating the detector current, the integrating amplifier being coupled to the detector element via a hold switch, and a timing circuit for generating a hold signal, SW HOLD , for operating the hold switch to couple the integrating amplifier to the detector element during a predetermined interval of the detector current.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of EP provisional application s/n06123851.5, filed Nov. 10, 2006, which is incorporated herein byreference. A related application is PCT IB2007/054472, “OscillatorElement for Photo Acoustic Detector,” filed Nov. 5, 2007, published asWO 2008/056312.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a photo acoustic detector for detecting aconcentration of a sample in a sample mixture, the photo acousticdetector comprising a light source for producing a light beam forexciting molecules of the sample, a light modulator for modulating thelight beam for generating pressure variations in the sample mixture, anamplitude of the pressure variations being a measure of theconcentration, a detector element for converting the pressure variationsinto a detector current, and a processing section for processing thedetector current to generate an output signal representing theconcentration.

BACKGROUND OF THE INVENTION

Such a photo acoustic detector is known from the United States patentapplication, published as US 2005/0117155. Said patent applicationdescribes a photo acoustic trace gas detector using a quartz tuning forkfor detecting the pressure variations. Light modulation is performedusing amplitude or wavelength modulation. After amplification by apre-amplifier, a lock-in amplifier mixes the detector signal with areference signal for acquiring an output signal. The reference signalfor the lock-in amplifier is taken from a signal for modulating thelight beam. The use of the quartz tuning fork for the detection of thepressure variations allows for a relatively compact photo acoustic tracegas detector.

An application of photo acoustic trace gas detectors is breath testing.Breath testing is a promising area of medical technology. Breath testsare non-invasive, user friendly and low cost. Prime examples of breathtesting are monitoring of asthma, alcohol breath testing and detectionof stomach disorders and acute organ rejection. First clinical trialsshow possible applications in the pre-screening of breast and lungcancer. These volatile biomarkers have typical concentrations in theparts per billion (ppb) range. Nitric oxide (NO) is a well known tracegas in the human breath, and elevated concentrations of NO can be foundin asthmatic patients. Currently, exhaled NO levels at ppbconcentrations can only be measured using expensive and bulky equipmentbased on chemiluminescence or optical absorption spectroscopy. Acompact, low-cost NO sensor forms an interesting device that can be usedto diagnose and monitor airway inflammation and can be used at thedoctor's office and for medication control at home.

It is a problem of the photo acoustic trace gas detector according to US2005/0117155 that the detector currents during trace gas detection areoften very low and easily dominated by electronic noise, which limitsthe trace gas detection at low concentrations.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a photo acoustic detectoraccording to the opening paragraph, which detector has a lower detectionlimit than state of the art detectors.

According to a first aspect of the invention, this object is achievedbecause the processing section comprises an integrating amplifier forintegrating the detector current, the integrating amplifier beingcoupled to the detector element via a hold switch, and a timing circuitfor generating a hold signal, SW_(HOLD), for operating the hold switchin order to couple the integrating amplifier to the detector elementduring a predetermined interval of a period of the detector current.

The detector element provides a detector current that oscillates at theresonance frequency of the pressure variations induced by the lightsource in the sample mixture. The operating of the hold switch in theintegrating amplifier results in sampling only a predetermined intervalduring a period of the detector current. The fixed interval is chosenshorter than the whole period, because the positive part of theoscillating detector current compensates the negative part and, as aconsequence, the integrated value of the detector current over a wholeperiod does not depend on the amplitude of the pressure variations.Integrating the detector current over a whole period does not provideinformation about the concentration of the sample in the sample mixture.By only integrating a shorter predetermined interval of the signal, thiscompensation does not occur and a higher amplitude results in a higherintegrated value. Each obtained signal sample is a measure of theconcentration of the sample. By adding the obtained signal samples formultiple periods of the detector current, the gain of the output signalis improved and lower sample concentration can be detected.

Preferably, the timing circuit is arranged for operating the hold switchrepeatedly by generating the hold signal, SW_(HOLD), as a periodicsignal with a frequency equal to the frequency of the detector currentand a duty cycle of 50%.

Preferably the detector current and the hold signal, SW_(HOLD), are inphase or in antiphase. By sampling 50% of each period of the detectorcurrent, only the positive part of the detector current or only thenegative part of the detector current is integrated. By adding only thepositive parts of multiple periods of the signal from the oscillatorelement, the gain and the signal to noise ratio of the detector is muchimproved, thereby resulting in a more sensitive photo acoustic detector.When the detector current and the hold signal SW_(HOLD) are not exactlyin phase or in antiphase, the gain of the detector decreases.

Preferably, the detector element is an oscillator element and the lightmodulator is arranged for modulating the light beam at a resonancefrequency of the oscillator element.

Due to its small bandwidth, such an oscillator element is less sensitiveto environmental acoustic noise. The detector current from such anoscillator is highly sinusoidal, which makes it very suitable for usewith the processing schemes described below.

In another embodiment of the photo acoustic detector, the timing circuitis arranged for operating the hold switch repeatedly by generating thehold signal, SW_(HOLD), as a periodic signal with a frequency equal toone third of the frequency of the detector current and a duty cycle of50%.

In this embodiment the detector current is integrated once every threeperiods and the fixed interval during which the detector current isintegrated comprises two positive and one negative part (or vice versa).One positive part compensates for the negative part and the secondpositive part contributes to the output signal. Also in this embodiment,the detector current and the hold signal, SW_(HOLD), are preferably inphase or in antiphase.

This embodiment also eliminates disadvantages of the above describedembodiment wherein the hold signal, SW_(HOLD), has a frequency equal tothe frequency of the detector current and a duty cycle of 50%. Thatembodiment, if combined with the use of an oscillator element fordetection of the pressure variations, shows the disadvantage that thehold switch switches at the same frequency as the resonance frequency ofthe detector signal. When the hold switch (typically a FET) switches,some small current flows through this switch at the resonance frequencyof the oscillator element. As a result the oscillator gets excited abit, causing an offset in the output signal. So even without laser lightand sample molecules, an offset at the output occurs. When switching thehold signal, SW_(HOLD), at a frequency equal to one third of thefrequency of the detector current the hold switch is not operated at theresonance frequency of the oscillator element and the oscillator elementdoes not resonate anymore on switching SW_(HOLD). As a result, theswitching does not influence the detector current.

In another embodiment, a similar effect is achieved by generating thehold signal, SW_(HOLD), as a periodic signal with a frequency equal tohalf of the frequency of the detector current and a duty cycle of 75%.

This embodiment has the additional advantage, that the time needed fortaking a sample is shorter than in the previous embodiment. This resultsin faster detection with the same signal to noise ratio, or equally fastdetection with a better signal to noise ratio. In another embodiment,the processing section is arranged for taking a first and a secondmeasurement by respectively generating a first and a second outputsignal, the hold signal, SW_(HOLD), used for the second measurementbeing phase shifted over half a period of the detector current, andcalculating an average output signal from the absolute values of thefirst and the second output signal.

In this embodiment the offset caused by the switching of the hold switchis averaged out. The first measurement gives a positive result and thesecond measurement gives a negative result, but both carry the sameoffset.

In yet another embodiment, the hold switch is coupled to the oscillatorelement via a buffer stage. The buffer stage results in some extra gain.The buffer stage also results in offset canceling, because there is nodirect coupling anymore between the detector element and the holdswitch.

In a preferred embodiment, the processing section further comprises aselect switch for copying an integrated voltage from the integratingamplifier to the output signal and a reset switch for resetting theintegrating amplifier and wherein the timing circuit is arranged forgenerating a select signal, SW_(SELECT), for operating the select switchand a reset signal, SW_(RESET), for successively operating the resetswitch and wherein the timing circuit is further arranged for generatingthe hold signal, SW_(HOLD), with a frequency of at least twice afrequency of the reset signal, SW_(RESET).

In this embodiment, the integrated detector currents of at least two,but preferably more, consecutive samples are summed. For a higher signalto noise ratio, more samples are collected before copying the capacitorcharge to the output signal and resetting the capacitor for starting anew measurement.

In a preferred embodiment of the photo acoustic detector the processingsection further comprises a post processing unit with a comparator forcomparing an integrated voltage from the integrating amplifier to apredetermined value, a reset pulse generator for, when the integratedvoltage reaches the predetermined value, providing a reset pulse,SW_(RESET), for closing a reset switch and resetting the integratingamplifier, and a timer for, when the integrated voltage reaches thepredetermined value, determining a total sampling time used for reachingthe predetermined value.

For small detector currents, more gain is needed to obtain sufficientsignal to noise ratio, which results in a longer integration time. Forlarge detector currents however, less gain is needed, which results in ashorter integration time. By adaptively calculating the total samplingtime, the signal to noise ratio can be kept sufficient and theintegration time can be kept as short as possible.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 schematically shows a prior art photo acoustic trace gasdetector,

FIG. 2 schematically shows a photo acoustic trace gas detector accordingto the invention,

FIG. 3 shows a collection of signals demonstrating the operation of thephoto acoustic trace gas detector according to FIG. 2,

FIG. 4 shows a collection of signals demonstrating the operation ofanother embodiment of the photo acoustic trace gas detector according toFIG. 2,

FIG. 5 schematically shows a preferred embodiment of the photo acoustictrace gas detector according to the invention,

FIG. 6 a shows an exemplary arrangement of the post processing unitcomprised in the embodiment of FIG. 5, and

FIG. 6 b shows a collection of signals demonstrating the operation ofthe embodiment shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a prior art photo acoustic detector. Thephoto acoustic trace gas detector described hereinafter detects tracegas concentrations in gas mixture, but the invention may also be appliedto detect tissue, fluid or solid samples in other sample mixtures. Thetrace gas detector 100 uses a laser diode 101 as a light source. Thewavelength of the laser light is chosen such that it can excite thetrace gas molecules. Alternatively, other types of laser sources orother light sources, capable of producing a light beam with sufficientenergy to excite the trace gas molecules may be used. A laser driver 102provides a driving signal for the laser diode 101. In this embodiment,the laser driver 102 also functions as a modulator for modulating thelight beam. The laser driver 102 comprises a DC source 121 for providinga DC signal and an AC source 122 for providing an AC signal. The DCsignal and the AC signal are combined in adder 123 and then provided tothe laser diode 101. As a result, the intensity of the light beamchanges in time, following a sinusoidal pattern. A higher intensity ofthe laser beam results in more molecules in the trace gas being excited,which leads to a higher temperature of the gas mixture. A largeramplitude of the driving signal results in larger temperaturefluctuations. A higher concentration of the trace gas also results inlarger temperature fluctuations. The temperature fluctuations causepressure variations or sound waves in the gas mixture. The pressurevariations are detected by a detector element, such as a microphone oran oscillator element 103. If the laser light is modulated at theresonance frequency of an oscillator element 103, the sound waves excitethe oscillator 103. Preferably, the oscillator element 103 is a crystaloscillator, such as a quartz tuning fork. Quartz tuning forks have ahigh sensitivity and operate at a high frequency. Furthermore, quartztuning forks are not very expensive because they are used on largescale, for example, for the manufacturing of digital watches.

Modulation of the intensity of the light beam may also be realized bymanipulating a light beam with a continuous intensity. It is, forexample, known to us a mechanical chopper for generating an intensitymodulated light beam from a continuous wave light beam.

In an alternative embodiment, the intensity of the light beam isconstant and the wavelength of the laser light is modulated. Thisembodiment takes advantage of the effect that only light in a specificrange of wavelengths is suited for exciting the trace gas molecules. Forwavelength modulation, the laser is modulated with the half of theresonance frequency of the oscillator 103. The oscillator 103 thenstarts to resonate at its resonance frequency (wavelength modulationdoubles the frequency).

The oscillating crystal oscillator 103 generates a small oscillatingdetector current I_(osc) with a frequency, equal to the resonancefrequency of the oscillator 103 and with an amplitude, proportional tothe trace gas concentration. A signal processing unit 106 processes thedetector current I_(OSC) to provide an output signal U_(OUT), indicativeof the trace gas concentration. The prior art processing unit comprisesa pre-amplifier 104 and a lock-in detector 105. The pre-amplifier 104amplifies this current I_(OSC). Lock-in detection does the generation ofthe actual output. The lock-in detector 105 mixes the amplified signalwith a reference signal that has the same phase as the amplified signal.The reference signal is derived from the AC laser signal. A trigger 107and a phase shifter 108 are used for providing the reference signal to amixer 113. The mixer 113 mixes the reference signal with the amplifiedsignal. The mixer output is low pass filtered, so a DC output U_(OUT)represents the detected trace gas concentration.

FIG. 2 schematically shows a photo acoustic trace gas detector 200according to the invention. According to the invention, thepre-amplifier 104 and the lock-in detector 105 are replaced by anintegrating amplifier with a special switching algorithm to do thepre-amplification and the generation of the output signal U_(OUT). Theactual mixing is done in the integrator, so external lock-in detectionis not needed anymore. The laser diode 101 is again supplied with an ACand DC current. The light induced pressure variations excite theoscillator 103 and the integrating amplifier amplifies the detectorcurrent I_(OSC). The integrator shown in FIG. 2 comprises an op-amp 110and a capacitor 111, interconnecting the output of the op-amp 110 withthe negative input of the op-amp 110. The integrator has 3 switches tobe controlled. A hold switch is used to store the oscillator currentI_(OSC) into a capacitor 111. This switch needs to be in phase withI_(OSC). A reset switch resets the integrator when needed and a selectswitch copies the integrated output value U_(INT) to the output. Atrigger 107 together with a phase shift circuit 108 is used to get theswitching signal SW_(HOLD) for the hold switch, in phase with I_(OSC). Atiming generator block 109 (normally a small piece of digital logic)generates the switching signals from the AC laser frequency. Every oncein a while (e.g. 10 or 100 times per second) the timing generator block109 generates a select signal SW_(SELECT) and the integrated signalU_(INT) is copied to the output. Directly after that, a reset signalSW_(RESET) resets the capacitor 111. So the gain also depends on thereset frequency. FIG. 3 shows a collection of signals demonstrating theoperation of the photo acoustic trace gas detector 200 according to FIG.2. In FIG. 3, the following signals are shown:

-   I_(OSC): The signal from the oscillator 103. The frequency of the    signal is the same as the frequency of the AC modulation frequency    of the amplitude modulation or, when wavelength modulation is used,    twice the AC modulation frequency of the wavelength modulation. The    amplitude of I_(OSC) is proportional to the concentration of the    trace gas. FIG. 3 shows one increase of the trace gas concentration.    At t=t₁ the trace gas concentration and the detector current I_(OSC)    roughly double.-   SW_(HOLD): The hold signal, SW_(HOLD), controls the hold switch.    When SW_(HOLD) is high, the switch is closed. When SW_(HOLD) is low,    the switch is open. A comparison of I_(OSC) and SW_(HOLD) shows that    only the negative parts of the detector current I_(OSC) are fed to    the capacitor 111. When I_(OSC) is positive, the hold switch is open    and I_(OSC) is not integrated by the capacitor 111. In this example,    the frequency of SW_(HOLD) is the same as the frequency of I_(OSC)    and the duty cycle is 50%.-   U_(INT): When the hold switch is closed and the detector current    I_(OSC) is fed into the capacitor 111, the voltage, U_(INT), at the    output of the op-amp 110 increases. When the hold switch is open,    U_(INT) remains constant. SW_(SELECT): Every once in a while (e.g.    10 or 100 times per second) the select signal, SW_(SELECT), is high    and the voltage U_(INT) is sampled by a sample and hold circuit 112.-   SW_(RESET): Directly after sampling U_(INT), a reset signal,    SW_(RESET), causes a reset switch to close and the integrator is    reset. Thereafter, the integrator starts integrating the detector    current I_(OSC) again and continues to do so until the next reset. A    high reset frequency results in a high sampling rate, but a    relatively low gain. A low reset frequency results in a lower    sampling rate and a higher gain. The gain thus also depends on the    reset frequency.-   U_(OUT): The sample and hold circuit provides the output signal,    U_(OUT). As shown in FIG. 3, the trace gas concentration increases    at t=t₁ (the amplitude of I_(OSC) increases) and the output signal,    U_(OUT), changes the first time that a new sample is taken    (SW_(SELECT) is high).

The embodiment shown with reference to FIG. 3 has the disadvantage thatthe hold switch switches at the same frequency as the resonancefrequency of the crystal oscillator. When the hold switch (typically aFET) switches, some small current flows through this switch at theresonance frequency of the crystal oscillator. As a result theoscillator gets excited a bit, causing an offset in U_(OUT). So evenwithout laser light and trace gas, an offset at the output occurs. Thisdisadvantage does not occur in the embodiment which is demonstrated withreference to FIG. 4. FIG. 4 shows a collection of signals demonstratingthe operation of another embodiment of the photo acoustic trace gasdetector 200 according to FIG. 2. In this event, the frequency ofSW_(HOLD) is one third of the frequency of I_(OSC) and the duty cycle is50%. In this embodiment the detector current is integrated once everythree periods and the fixed interval during which the detector currentis integrated comprises two negative and one positive part. The firstnegative part compensates for the positive part and the second negativepart contributes to the output signal U_(OUT). Because the hold switchis not operated at the resonance frequency of the crystal oscillator,the switching does not influence the detector current I_(OSC).

A similar effect is achieved by generating the hold signal, SW_(HOLD),with a frequency equal to half of the frequency of the detector currentI_(OSC) and a duty cycle of 75%. This embodiment has the additionaladvantage, that the time needed for taking a sample is shorter than inthe previous embodiment. This results in faster detection with the samesignal to noise ratio, or equally fast detection with a better signal tonoise ratio.

In principle all duty cycles of less than 100% for SW_(HOLD) enableintegrating the detector current I_(OSC). However, a duty cycle of, forexample, 1% or 99% would result in an improved gain only if theintegration time is very long. Also for the frequency of SW_(HOLD), alot of different values may be selected. In all embodiments, it isimportant that a suitable combination of the frequency and duty cycle ofSW_(HOLD) and the frequency of SW_(SELECT) and SW_(RESET) is selected.Some examples of suitable combinations are described above.Alternatively, the frequency of SW_(HOLD) may, e.g., be 99% of thefrequency of the light modulation, which results in a low frequency mixsignal. This low frequency mix signal may also be used as a measure forthe sample concentration, if it is sampled at a suitable frequency.

In another embodiment, the processing section is arranged for generatinga first and a second output signal, the hold signal, SW_(HOLD), used forobtaining the second output signal being 180° phase shifted with respectto the detector current, and calculating an average output signal fromthe absolute values of the first and the second output signal. In thisembodiment the offset caused by the switching of the hold switch and theresulting excitation of the crystal oscillator is averaged out. Thefirst measurement gives a positive result and the second measurementgives a negative result, but both carry the same offset.

In yet another embodiment, the hold switch is coupled to the crystaloscillator via a buffer stage. The buffer stage results in some extragain and prevents excitation of the detector by current from the holdswitch. The gain of the buffer stage is kept sufficiently small so thenoise current of the buffer stage stays far below the detector noisecurrent.

FIG. 5 schematically shows a preferred embodiment of the photo acoustictrace gas detector 200 according to the invention. In this embodimentthe processing section 106 comprises a post processing unit 112 forcomparing the integrated voltage U_(INT) from the capacitor 111, to apredetermined value, determining a total sampling time used for reachingthe predetermined value when the integrated voltage reaches thepredetermined value and outputting the total sampling time as the outputsignal. For small detector currents, more gain is needed to obtainsufficient signal to noise ratio, which requires a longer integrationtime. For large detector currents however, less gain is needed,requiring a shorter integration time. By adaptively calculating thetotal sampling time, the signal to noise ratio can be kept sufficientand the integration time can be kept as short as possible.

FIG. 6 a shows an exemplary arrangement of the post processing unit 112comprised in the embodiment of FIG. 5. The post processing unit 112comprises a comparator 201 for comparing the integrated voltage U_(INT)from the capacitor 111, to the predetermined value, U_(COMP). When theintegrated voltage, U_(INT), reaches the predetermined value, U_(COMP),a reset pulse generator 202 provides a pulse, SW_(RESET), for closing areset switch and discharging the capacitor 111. The output SW_(RESET) ofthe reset pulse generator 202 is also provided to a timer 203 for, whenthe integrated voltage reaches the predetermined value, determining atotal sampling time used for reaching the predetermined value. When thetrace gas concentration is higher, the predetermined value, U_(COMP), isreached sooner and the time between two reset pulses will be shorter.The ‘time to reset’ thus is indicative of the trace gas concentration.

FIG. 6 b shows a collection of signals demonstrating the operation ofthe embodiment shown in FIG. 5. Reset pulses, RW_(RESET), are providedwhen U_(INT), reaches U_(COMP). After an increase of the amplitude ofthe oscillator current, I_(OSC), the sampling time value, t_(OUT),changes as soon as U_(INT) reaches the predetermined value, U_(COMP),for the first time. Smaller values for the sampling time value, t_(OUT),relate to higher trace gas concentrations. For small detector currents,more gain is needed to obtain sufficient signal to noise ratio, whichresults in a longer integration time. For large detector currentshowever, less gain is needed, which results in a shorter integrationtime. By adaptively calculating the total sampling time, the signal tonoise ratio can be kept sufficient and the integration time can be keptas short as possible.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the claims enumerating several means,several of these means may be embodied by one and the same item ofhardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. A photo acoustic detector for detecting a concentration of a samplein a sample mixture, the photo acoustic detector comprising: a lightsource for producing a light beam for exciting molecules of the sample,a light modulator for modulating the light beam for generating pressurevariations in the sample mixture, an amplitude of the pressurevariations being a measure of the concentration, a detector element forconverting the pressure variations into a detector current, and aprocessing section for processing the detector current to generate anoutput signal representing the concentration, wherein the processingsection comprises: an integrating amplifier for integrating the detectorcurrent, the integrating amplifier being coupled to the detector elementvia a hold switch, and a timing circuit for generating a hold signal,SWHOLD, for operating the hold switch in order to couple the integratingamplifier to the detector element during a predetermined interval of aperiod of the detector current, wherein the predetermined interval isshorter than the period of the detector current.
 2. A photo acousticdetector for detecting a concentration of a sample in a sample mixture,the photo acoustic detector comprising: a light source for producing alight beam for exciting molecules of the sample, a light modulator formodulating the light beam for generating pressure variations in thesample mixture, an amplitude of the pressure variations being a measureof the concentration, a detector element for converting the pressurevariations into a detector current, and a processing section forprocessing the detector current to generate an output signalrepresenting the concentration, wherein the processing sectioncomprises: an integrating amplifier for integrating the detectorcurrent, the integrating amplifier being coupled to the detector elementvia a hold switch, and a timing circuit for generating a hold signal,SWHOLD, for operating the hold switch in order to couple the integratingamplifier to the detector element during a predetermined interval of aperiod of the detector current, wherein the timing circuit is configuredfor operating the hold switch repeatedly by generating the hold signal,SWHOLD, as a periodic signal with a frequency equal to a frequency ofthe detector current and a duty cycle of 50%.
 3. The photo acousticdetector according to claim 1, wherein the detector element comprises anoscillator element, and wherein the light modulator is configured formodulating the light beam at a resonance frequency of the oscillatorelement.
 4. The photo acoustic detector according to claim 3, whereinthe oscillator element comprises a crystal oscillator.
 5. A photoacoustic detector for detecting a concentration of a sample in a samplemixture, the photo acoustic detector comprising: a light source forproducing a light beam for exciting molecules of the sample, a lightmodulator for modulating the light beam for generating pressurevariations in the sample mixture, an amplitude of the pressurevariations being a measure of the concentration, a detector element forconverting the pressure variations into a detector current, and aprocessing section for processing the detector current to generate anoutput signal representing the concentration, wherein the processingsection comprises: an integrating amplifier for integrating the detectorcurrent, the integrating amplifier being coupled to the detector elementvia a hold switch, and a timing circuit for generating a hold signal,SWHOLD, for operating the hold switch in order to couple the integratingamplifier to the detector element during a predetermined interval of aperiod of the detector current, wherein the timing circuit is configuredfor operating the hold switch repeatedly by generating the hold signal,SWHOLD, as a periodic signal with a frequency equal to one third of thefrequency of the detector current and a duty cycle of 50%.
 6. A photoacoustic detector for detecting a concentration of a sample in a samplemixture, the photo acoustic detector comprising: a light source forproducing a light beam for exciting molecules of the sample, a lightmodulator for modulating the light beam for generating pressurevariations in the sample mixture, an amplitude of the pressurevariations being a measure of the concentration, a detector element forconverting the pressure variations into a detector current, and aprocessing section for processing the detector current to generate anoutput signal representing the concentration, wherein the processingsection comprises: an integrating amplifier for integrating the detectorcurrent, the integrating amplifier being coupled to the detector elementvia a hold switch, and a timing circuit for generating a hold signal,SWHOLD, for operating the hold switch in order to couple the integratingamplifier to the detector element during a predetermined interval of aperiod of the detector current, wherein the timing circuit is configuredfor operating the hold switch repeatedly by generating the hold signal,SWHOLD, as a periodic signal with a frequency equal to half of thefrequency of the detector current and a duty cycle of 75%.
 7. The photoacoustic detector according to claim 2, wherein the processing sectionis configured for: taking a first measurement and a second measurementby respectively generating a first output signal and a second outputsignal, the hold signal, SWHOLD, used for the second measurement beingphase shifted over half a period of the detector current, andcalculating an average output signal from absolute values of the firstoutput signal and the second output signal.
 8. The photo acousticdetector according to claim 1, wherein the hold switch is coupled to thedetector element via a buffer stage.
 9. A photo acoustic detector fordetecting a concentration of a sample in a sample mixture, the photoacoustic detector comprising: a light source for producing a light beamfor exciting molecules of the sample, a light modulator for modulatingthe light beam for generating pressure variations in the sample mixture,an amplitude of the pressure variations being a measure of theconcentration, a detector element for converting the pressure variationsinto a detector current, and a processing section for processing thedetector current to generate an output signal representing theconcentration, wherein the processing section comprises: an integratingamplifier for integrating the detector current, the integratingamplifier being coupled to the detector element via a hold switch, and atiming circuit for generating a hold signal, SWHOLD, for operating thehold switch in order to couple the integrating amplifier to the detectorelement during a predetermined interval of a period of the detectorcurrent, wherein the processing section further comprises a selectswitch for copying an integrated voltage from the integrating amplifierto the output signal and a reset switch for resetting the integratingamplifier, wherein the timing circuit is configured for generating aselect signal, SWSELECT, for operating the select switch and a resetsignal, SWRESET, for successively operating the reset switch, andwherein the timing circuit is further configured for generating the holdsignal, SWHOLD, with a frequency of at least twice a frequency of thereset signal, SWRESET.
 10. A photo acoustic detector for detecting aconcentration of a sample in a sample mixture, the photo acousticdetector comprising: a light source for producing a light beam forexciting molecules of the sample, a light modulator for modulating thelight beam for generating pressure variations in the sample mixture, anamplitude of the pressure variations being a measure of theconcentration, a detector element for converting the pressure variationsinto a detector current, and a processing section for processing thedetector current to generate an output signal representing theconcentration, wherein the processing section comprises: an integratingamplifier for integrating the detector current, the integratingamplifier being coupled to the detector element via a hold switch, and atiming circuit for generating a hold signal, SWHOLD, for operating thehold switch in order to couple the integrating amplifier to the detectorelement during a predetermined interval of a period of the detectorcurrent, wherein the processing section further comprises a postprocessing unit including: a comparator for comparing an integratedvoltage from the integrating amplifier to a predetermined value, a resetpulse generator for, when the integrated voltage reaches thepredetermined value, providing a reset pulse, SWRESET, for closing areset switch and resetting the integrating amplifier, and a timer for,when the integrated voltage reaches the predetermined value, determininga total sampling time used for reaching the predetermined value.
 11. Thephoto acoustic detector of claim 3, wherein the oscillator elementcomprises a quartz tuning fork.